A woman is found strangled in her shop. Shortly after, another murder is committed. Perhaps the most terrifying is that the both girls' name is Sonja. The police is convinced it's the same killer and at each scene is heard a woman singing the same song. A song which is soon dubbed the 'murder melody.'

Gull-Maj Norin
as Odette Margot / Sonja

Poul Reichhardt
as Max Stenberg

Angelo Bruun
as Louis Valdini

Else Petersen
as Mrs. Baunsø

Ib Schønberg
as Theatre director Perm

Peter Nielsen
as Assistant Detective Baunsø

Karen Poulsen
as Flora Kristiansen

Petrine Sonne
as Sonja Bohman

Charles Wilken
as Boarding House Host

Per Buckhøj
as Markussen